An exemplary nuclear reactor, such as a boiling water reactor (BWR), includes a reactor core disposed in a reactor pressure vessel, with the pressure vessel being in turn disposed in a containment vessel. The reactor core includes a plurality of nuclear fuel rods configured in bundles to generate heat which is transferred to water recirculating therein for generating steam to power a steam turbine-generator, for example.
Reactivity in the core is typically controlled by a plurality of control rods typically extending vertically upwardly therein from the bottom of the pressure vessel. Conventional control rod drives (CRDs) are mounted below the pressure vessel in a lower drywell region of the containment vessel for selectively inserting the control rods into the core and withdrawing the control rods therefrom for controlling reactivity. The lower drywell is defined by an annular pedestal wall which is used to support the pressure vessel.
The containment vessel is typically a concrete structure having an inner steel liner designed for containing the expected elevated pressure therein in the event of a nuclear accident, as well as preventing significant nuclear radiation released therefrom. The lower drywell being disposed below the pressure vessel typically includes one or more water sumps for collecting leaking water which is then conventionally removed therefrom by pumps. In the event of a severe accident in which the reactor core melts to form hot, molten core debris known as corium, the corium must be suitably contained within the containment vessel without appreciably damaging the containment vessel or the reactor pedestal. As the molten debris spreads over the floors of the containment vessel, the flow thereof will be fairly uniform in thickness if there are no obstructions and the floors are uniformly level. In this way, the molten corium may be allowed to uniformly cool and solidify. Since the molten corium includes molten reactor fuel which generates heat even after the reactor is shutdown, it may reliquify if suitable cooling thereof is not maintained.
For example, requirements for an advanced light water reactor include providing a suitable floor area beneath the pressure vessels for limiting the thickness of molten corium following an accident to promote the cooling thereof. However, since the water sumps are located below the pressure vessel, the molten corium may flow and collect therein at a substantially greater thickness than that over the remainder of the drywell floor, the increased thickness of the molten corium in the sumps decreases its ability to be cooled and increases the uncertainty of solidifying the molten corium, preventing it from reliquifying due to the heat being generated therein, and preventing it from interacting with the floor material thereby releasing radioactive material and noncondensable gases.